Guidewires are used for directing catheters to precise locations within passageways of living bodies. These passageways are often of small inside diameter (as small as 0.020 inch), incorporate many branches and present tortuous, curved paths. The ideal guidewire must be break-resistant, flexible, kink-resistant, have a smooth and lubricious surface, have a minimal outside diameter, provide good torque characteristics and offer good column strength to allow the guidewire to be pushed through complex passageways like the vascular system.
A guidewire is typically inserted into a guiding catheter which was previously placed into the vascular system through a cannula device and pushed through the vascular system to the desired location by routing through the appropriate branches. Careful manipulation of the guidewire past the distal end of the guiding catheter is required while viewing the passage of the guidewire radiographically. After the distal tip of the guidewire is in the desired position, a catheter is inserted over the guidewire and moved along the length of the guidewire to the desired position.
Guidewires typically take the form of a tightly wound spring which is constructed of very fine wire tightly wrapped into a helically wound coil spring in which adjacent turns typically contact each other. Guidewires are generally available as small as 0.014 inch outside diameter formed from round-section wire of diameter as small as 0.002 inches. Wires of essentially square and rectangular cross-section have been used as well, as have round wires with their outer surface ground flat in order that the guidewire may present a flat surface to the tissue walls.
In use, breakage of the helically wound guidewire is known to occur on occasion, resulting in the separation and loss of the distal end of the guidewire. Surgical intervention is often required to retrieve the lost end. It is common practice to employ a safety wire oriented along the axis of the guidewire to prevent the loss of a broken distal guidewire portion. This is accomplished by suitably attaching (welding, brazing, etc.) the tip of the safety wire to the tip of the guidewire. The use of safety wires has reduced the frequency of breakage and loss of the distal end, but has not eliminated it, apparently due to breakage of the safety wire or its attachment to the guidewire during the same trauma responsible for the breakage of the guidewire.
Some guidewires do not use a safety wire within the distal end portion of the device so that the device tip may be as flexible as possible. It is this distal end portion that is most commonly broken and lost.
Breakage of these devices generally appears to follow the same pattern. In attempting to pass the guidewire around a sharp bend or through an obstructed passageway, the tip of the device becomes trapped. The operator generally attempts to free the device with rotary, extension (compression) and traction (tensile) forces. The application of an excessive traction force results in uncoiling of the guidewire. If this traction force is then relieved momentarily, the uncoiled length of guidewire tries to recover at least some of its previously coiled form. The application of a rotary force to the partly uncoiled wire appears to result in tangling of the wire as the uncoiled wire loops or crosses over itself in one or more places. Continued application of rotary and/or traction forces causes kinking of the tangled wire, which quickly results in breakage of the wire at the location of a kink.
Uncoiling is herein meant to mean extension of the coil spring beyond its elastic limit.
Guidewires have been available for some time with plastic coatings, most frequently of polytetrafluoroethylene (hereinafter PTFE). This is done so as to present a smooth, lubricious and inert surface to the vessel wall. Such coatings have been applied to the outer surface of the guidewire coil and have also been applied to the circumferential surface of the wire before winding the wire into a coil so that the entire circumference of the wire surface is coated.
While previous PTFE coated guidewires have provided improved guidewire performance, these previous coatings still have significant shortcomings. PTFE coatings have typically been applied by either dip-coating the wire in a liquid dispersion of PTFE or by covering with PTFE heat-shrink tubing. Either method may be used to coat the circumferential surface of the wire before winding into a guidewire or to coat the outer surface of the already wound guidewire. Dip-coatings may be applied more thinly than heat-shrink tubing, however, such dip-coatings are prone to flaking during manipulation of the guidewire. Such flaking presents an undesirable risk of contamination to the patient. Heat-shrink coatings generally are not vulnerable to flaking, however, the use of such coatings results in a guidewire of increased outside diameter due to the increased thickness of the heat-shrink coating. Additionally, heat shrink tubing covering the distal end of small diameter guidewires significantly restricts flexibility.